1. Field of the Invention
The present invention relates to an optical waveguide such as a planar lightwave circuit and a fabricating method thereof, and in particular, to an optical waveguide and a fabricating method thereof in which a core for transmitting a light is formed by deposition.
2. Related Art
As demands of various communications such as the Internet are increasing, communication networks are expanding, so that the node configuration is tend to be complicated and each node is required to perform high-level processing of various kinds. In such a situation, attention is directed to an optical switching technique capable of performing processing without converting optical signals into electric signals. In order to realize optical communication systems, a planar lightwave circuit (PLC) technique suitable for integration is promising.
The planar lightwave circuit technique suitable for integration is usually configured to transmit light through multiple passive optical waveguides such as an optical wavelength multiplexer/demultiplexer, a dispersion compensator, a gain equalizer for optical fiber amplifier, an optical switch, and the like. As a result, the propagation length of the light becomes longer, so that it is indispensable to suppress propagation loss in these passive devices. However, there is a fact that in an optical waveguide having a planar lightwave circuit, the propagation loss is extremely large comparing with that of an optical fiber. Accordingly, if a number of these optical waveguides are connected in series to be used, a serious problem is caused in its propagation loss.
FIG. 1 shows the main part of of the cross-sectional structure of an optical waveguide having a conventional planar lightwave circuit. On a substrate 11, there is formed a lower cladding 12 constituting the lower half of the cladding in a prescribed thickness. On the lower cladding 12, a core 13 is formed, and further an upper cladding 14 is formed so as to cover around the core 13.
In the optical waveguide of this kind, although the surface of the substrate has a sufficient smoothness, the surface becomes to have fine irregularities when the lower cladding 12 is formed above a certain thickness. Then, when the core 13 is formed on the lower cladding 12, the Mie scattering is caused at the boundary between them. The Mie scattering means light scattering with the irregularities on the surface of an object in micron size, which cannot be disregarded with reference to the wavelength. The scattering loss at the boundary between the lower cladding 12 and the core 13 is a serious cause of a propagation loss. To cope with this, the surface of the lower cladding 12 is optically polished so as to be the smooth face, and the core 13 is formed after this polishing step, conventionally.
However, with the optical polishing, the fabricating efficiency of the optical waveguides reduces, so as to deteriorate mass productivity. Further, it is required to perform product management so that the surface roughness by the optical polishing becomes uniform in respective optical waveguides, which troubles the managing operation. To cope with this, a technique in which a smoothing layer is formed at the boundary between the cladding and the core so as to prevent light scattering is proposed as a first proposal in, for example, Japanese Patent Application Laid-open No.6-235,8375 (paragraphs [0017], [0018] and FIG. 4A).
FIG. 2 shows the main part of the cross-sectional structure of an optical waveguide according to the first proposal. A glass substrate 21 serving as a cladding of the optical waveguide has a refractive index of n1, and fine irregularities 22 exist on the surface thereof. The glass substrate 21 is dried and then put into a firing furnace, then a sol-gel glass material is fired at a prescribed temperature and hardened, whereby a smoothing layer 23 with a smooth surface is formed integral with the glass substrate 21. The refractive index n2 of the smoothing layer 23 is almost equal to the refractive index n1. On the surface of the smoothing layer 23, corning glass 7059 (product name) is deposited by sputtering, so as to form a core 24 with a refractive index n3 which is larger than the refractive index n1 and the refractive index n2.
In the optical waveguide shown in FIG. 2, the sol-gel glass material is entered by spin coating or the like into the fine irregularities 22 on the surface of the glass substrate 21 so as to smooth the surface. However, the optical waveguide formed in this way causes a problem of large polarization dependency due to a warp of the substrate 21 influenced by a distortion stress which is based on the difference of coefficient of thermal expansion of the smoothing layer 23 and the core 24 and for the glass substrate 21.